I. Field of the Invention
This invention relates to digital-to-analog converters, and more particularly to D.C. offset error correction in digital-to-analog converters.
II. Description of the Related Art
Digital-to-analog converters, commonly referred to as "DACs" or "D-to-A" converters, are used to translate information from the digital domain to the analog domain. DACs typically transform digital signals into a range of analog values. DACs represent a limited number of different digital input codes by a corresponding number of discrete analog output values. Examples of input code formats accommodated by existing DACs include simple binary, two's complement binary, and binary-coded decimal. A number of techniques for implementing digital-to-analog converters are well known in the art.
Digital-to-analog converters are used in a wide variety of applications including digital wireless communications. For example, DACs are used in digital wireless cellular telephones to convert digital voice signals to "baseband" analog signals (i.e., signals having frequencies near D.C.). FIGS. 1a and 1b show a block diagram of an exemplary digital wireless cellular telephone 900 that utilizes DACs to convert digitally encoded voice signals into filtered baseband analog signals. The cellular telephone 900 is manufactured in accordance with the TIA specification entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System," TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as the "IS-95 specification."
As shown in FIGS. 1a and 1b, the exemplary digital cellular telephone 900 primarily comprises a user interface section 916, a mobile station modem (MSM) application specific integrated circuit (ASIC) 914, a baseband analog ASIC 912, receive and transmit amplifiers 902 and 904 respectively, an upconverter 918, a power amplifier and driver 920, an antenna 906, a duplexor 908 and a low-noise amplifier (LNA) and mixer circuit 910. The cellular telephone 900 and its component parts are described in more detail in a related commonly assigned U.S. Pat. No. 5,880,631, issued on Mar. 9, 1999, entitled "High Dynamic Range Variable Gain Amplifier," which is hereby incorporated by reference. An understanding of the function and operation of many of the components of the cellular telephone 900 are not essential to understanding the present invention and therefore are not described herein. However, a brief description of the MSM 914 and baseband analog ASIC 912 is useful in understanding one exemplary application and operating environment for the present invention.
The MSM 914 performs a variety of functions for the cellular telephone 900 including voice coding, decoding, interleaving, data modulation, spreading and filtering. For example, when information is transmitted from the telephone 900 to a CDMA base station ("reverse link" transmissions), voice information is first coded by the vocoder 950 and transferred to the modulator interleaver circuit 952 where the data is encoded, interleaved, modulated, spread and filtered. The digitized and modulated data is supplied to a pair of DACs 954, 956 in the baseband analog ASIC 912 (FIG. 1b) for further processing. The MSM 914 provides a baseband modulated digital representation of the CDMA waveform to the DACs 954 and 956 in the baseband analog ASIC 912. The frequency range of the baseband digital signals is between D.C. (or 0 Hz) and approximately 630 kHz. The baseband analog ASIC 912 (largely due to the operation of the DACs 954, 956) converts the modulated digital data received from the MSM 914 into baseband analog signals. The baseband analog ASIC 912 filters the baseband analog signals generated by the DACs 954, 956 and "upconverts" the filtered signals to an analog intermediate frequency (IF) signal. The IF signal is supplied to the transmit automatic gain control (AGC) amplifier 904 and further processed for eventual transmission to a wireless base station.
A better understanding of the operation of the DACs 954, 956 can be obtained by describing the transmit section of the baseband analog ASIC 912 in more detail. One embodiment of the transmit section 100 of the baseband analog ASIC 912 of FIG. 1b is shown in FIG. 2. As shown in FIG. 2, the transmit section primarily comprises a pair of transmit DACs 102 (one each for the in-phase modulated baseband digital signals (I) and the quadrature-phase modulated baseband digital signals (Q)), a pair of CDMA filters 104, 106, and a transmit up-converter circuit 108. The well known quadrature modulation scheme preferably is used to up-convert to the IF frequency in the CDMA path of the transmit section 100 shown in FIG. 2. Therefore, two DACs are needed to perform the digital-to-analog conversion of the baseband digital signals received from the MSM ASIC 914. The IDAC 110 converts the received baseband digital in-phase signals to baseband analog in-phase signals. Similarly, the QDAC 112 converts the received baseband digital quadrature-phase signals to baseband analog quadrature-phase signals. In the embodiment shown in FIG. 2, the transmit DACs 102 have differential outputs to reduce the detrimental effects caused by external noise that may be generated elsewhere on the baseband analog ASIC 912.
The I and Q channel CDMA filters 104, 106 remove unwanted noise that is generated by the DACs 110 and 112, respectively. The CDMA filters 104, 106 comprise anti-alias filters which perform a smoothing function on the baseband analog signals generated by the transmit DACs 102 and thereby remove any high frequency components introduced by the DACs 102. Similar to the transmit DACs 102, the CDMA filters 104, 106 have differential outputs as shown in FIG. 2. The outputs of the CDMA filters 104, 106 are input to the transmit up-converter 108 which converts the baseband analog signals to an IF frequency for further processing and eventual transmission to a CDMA base station.
Disadvantageously, the transmit section 100 shown in FIG. 2 introduces errors which are manifest as added D.C. offsets (referred to hereafter as "offset induced errors") in the transmit signals of interest before the signals are output to the remainder of the cellular telephone circuitry. In particular, and referring again to FIG. 2, the offset induced errors can be imposed upon the transmit signals by the transmit DACs 102 and by active components in the CDMA filters 104 and 106. Because the CDMA filters 104 and 106 can be relatively complex the induced offset errors can be significant. Disadvantageously, the offset errors introduced into the signal path, and specifically into the input of the mixers 114, 116, can cause a carrier signal to appear in the IF signal generated at the output of the transmit up-converter circuit 108. To meet certain carrier suppression specifications it is necessary to reduce or eliminate the offset induced errors introduced by the transmit section 100. Unfortunately, the offset induced errors have proven difficult to eliminate in the past. Because the magnitude of the offsets vary widely depending upon the operational characteristics (i.e., voltage, temperature, etc.) of the baseband analog ASIC 912 the errors have proven difficult to eliminate. Therefore, a need exists for a method and apparatus which can reduce or eliminate the D.C. offset errors that appear at the input of the transmit mixers 114, 116.
A prior art approach at reducing the D.C. offsets is shown in FIG. 3. The prior art uses a fuse-based D.C. offset error correction circuit 120 to reduce the errors produced at the output of the CDMA filters 104, 106. The error correction circuit 120 primarily comprises a series of fuses and a relatively small DAC which is capable of adding an error adjustment to the signals at the input of the mixers 114, 116. The error correction circuit allows designers to measure the D.C. offset at the output of the filters under selected nominal conditions. Using well known fuse trimming techniques, fuses in the correction circuit 120 are blown until the errors are reduced to zero under the selected nominal conditions. Disadvantageously, this technique provides a static error correction solution. Once the fuses are blown, the errors can not be corrected under the varying operational conditions of the ASIC 912. For example, as the voltage and temperature of the ASIC 912 varies over time, D.C. offsets would be introduced despite the static settings of the correction circuit 120. Devices that were once usable under the nominal conditions at which the fuses were blown become unusable under some operating environments, thus adversely affecting the yield characteristics of the baseband analog ASIC 912.
Further, the prior art approach shown in FIG. 3 disadvantageously introduces an additional manufacturing and testing step into the fabrication of the ASIC 912. Using the prior art approach of FIG. 3, the manufacturer of the ASIC 912 must measure the offset errors, trim fuses to eliminate the offset errors, and test the results to ensure the fuses are trimmed properly. This process adds additional time to the fabrication of the ASIC 912 and consequently adds to the manufacturing cost of the ASIC. Therefore, an improved D.C. offset error correction method and apparatus is needed which does not require the use of fuses or fuse trimming technique. Further, an improved error correction method and apparatus is needed which dynamically monitors and corrects the errors introduced by the transmit section 100 under all operating conditions under which the ASIC 912 must operate.
Another technique for reducing D.C. offset errors is shown in FIG. 4. As shown in FIG. 4, an analog feedback loop correction circuit 122 is used to measure and suppress the D.C. offset errors produced at the output of the CDMA filters 104 and 106. The analog feedback loop 122 includes analog filters that distinguish the D.C. offset errors from the analog signals of interest. The feedback loop also includes integrators disposed to integrate the D.C. offset errors across integrating capacitors. By selecting the gains of the integrators appropriately, the integrators generate D.C. cancellation signals that are nominally equal to the undesired D.C. errors introduced into the signal path by the CDMA filters and the transmit DACs 102. The D.C. cancellation signals are added to the analog signals generated by the transmit DACs 102 thereby eliminating undesired D.C. feed-through. A more detailed description of this prior art approach (in the context of a received RF signal path) is given with reference to FIGS. 9 and 10 of U.S. Pat. No. 5,617,060, issued on Apr. 1, 1997 to Wilson et al. and assigned to the owner of the present invention, which is hereby incorporated by reference.
Disadvantageously, the analog feedback loop has proven very difficult to implement in an ASIC device. The analog signals of interest generated at the output of the CDMA filters 104, 106 have levels which are very close to D.C. Therefore, the corner frequency of the filters used to differentiate the D.C. offset errors from the signals of interest must be very low. Because the corner frequency (w.sub.pole) is proportional to the transconductance (g.sub.m) divided by the capacitance (C), the transconductance g.sub.m must be constrained to be very small or, alternatively, the value of C must be made to be relatively large. Unfortunately, the value of g.sub.m is very difficult to control and there is a limit to how small the transconductance can be made. In addition, physical and cost constraints limit how large the value of C can be made in an integrated circuit environment (large capacitors occupy large areas of an integrated circuit and therefore increase the costs of the integrated circuit). One possible solution is to implement C using a component disposed outside of the integrated circuit, however this approach creates undesirable circuit board current leakages. Therefore, it is desirable to provide a D.C. correction method and apparatus which is easily implemented in an integrated circuit, which does not require the use of fuse trimming, and which can dynamically and flexibly monitor and correct D.C. offsets as they are introduced. The present invention provides such a D.C. correction method and apparatus.